Some embodiments described herein relate to electromagnetic machines and more particularly to structures for an electronic machine having tension and compression components.
Permanent magnet electromagnetic machines (referred to as “permanent magnet machines” or electromagnetic machines” herein) utilize magnetic flux from permanent magnets to convert mechanical energy to electrical energy or vice versa. Various types of permanent magnet machines are known, including axial flux machines, radial flux machines, and transverse flux machines, in which one component rotates about an axis or translates along an axis, either in a single direction or in two directions (e.g., reciprocating, with respect to another component). Such machines typically include windings to carry electric current through coils that interact with the flux from the magnets through relative movement between the magnets and the windings. In a common industrial application arrangement, the permanent magnets are mounted for movement (e.g., on a rotor or otherwise moving part) and the windings are mounted on a stationary part (e.g., on a stator or the like). Other configurations, typical for low power, inexpensive machines operated from a direct current source where the magnets are stationary and the machine's windings are part of the rotor (energized by a device known as a “commutator” with “brushes”) are clearly also available, but will not be discussed in detail in the following text in the interest of brevity.
In an electric motor, for example, current is applied to the windings in the stator, causing the magnets (and therefore the rotor) to move relative to the windings, thus converting electrical energy into mechanical energy. In a generator, application of an external force to the generator's rotor causes the magnets to move relative to the windings, and the resulting generated voltage causes current to flow through the windings—thus converting mechanical energy into electrical energy. In an AC induction motor, the rotor is energized by electromagnetic induction produced by electromagnets that cause the rotor to move relative to the windings on the stator, which are connected directly to an AC power source and can create a rotating magnetic field when power is applied.
Surface mounted permanent magnet machines are a class of permanent magnet machines in which the magnets are mounted on a ferromagnetic structure, or backing, commonly referred to as a back iron. Such machines are generally the lowest cost and lightest weight permanent magnet machines, but they typically suffer from limitations in performance that can be traced to a variety of design concerns. One such design concern is the size of the air gap between the stator and the rotor, as the electromagnetic efficiency of such machines tends to improve as the air gap size is reduced. Maintaining a constant air gap size is also important, both to avoid a collision between the rotor and the stator and to avoid unwanted currents, flux effects, and other load-related losses caused by eccentricities in the air gap. Consistency in air gap size is typically achieved by ensuring that the machine's stator and rotor (and any supporting structure) are stiff enough to withstand expected outside forces during assembly and operation. Significant violations of air gap size, such as where the air gap is nearly closed or is closed altogether, can be dangerous or destructive to equipment and personnel, particularly if the air gap is compromised during operation of the electromagnetic machine.
As the size of an electromagnetic machine increases (e.g., as known in wind power generation), dependence on structural stiffness to ensure that a minimum air gap clearance is maintained can become costly and/or can affect the overall efficiency of the machine due to the weight of the required structure. For example, generators of direct drive wind turbines tend to be large in diameter, ring like structures capable of handling large amounts of torque at low revolutions per minute. Such generators typically rely on a very stiff structure in torsion, with equally stiff responses to forces applied in the radial and axial directions. Such an approach is even more prevalent in an iron core permanent magnet generator where a small air gap is competing with high attractive forces between the rotor and the iron core stator from the magnets.
In an air core permanent magnet machine having no attractive forces between the stator and the rotor, the structure of the machine can be softer and lighter. For example, the structure can be soft axially and angularly, but stiff in torsion (or azimuthally). In such an air core permanent magnet machine, it may be desirable to allow the generator outer support member to deform axially, while maintaining a desired amount of torsional stiffness and/or its resistance to axial, radial and/or rotational deflections. Thus, a need exists for improved apparatus and methods to increase the structural efficiency of an electromagnetic machine and/or improve the ability of the electromagnetic machine to resist deflection in a variety of different directions.